The present invention relates to a process for converting hydrocarbons, e.g. naphtha, into olefins and preferably also into BTX. More in detail, the present invention relates to an integrated process based on a combination of hydrocracking, thermal and dehydrogenation to convert naphtha into olefins and preferably also into BTX as well.
U.S. Pat. No. 4,137,147 relates to a process for manufacturing ethylene and propylene from a charge having a distillation point lower than about 360 DEG C. and containing at least normal and iso-paraffins having at least 4 carbon atoms per molecule, wherein: the charge is subjected to a hydrogenolysis reaction in a hydrogenolysis zone, in the presence of a catalyst, (b) the effluents from the hydrogenolysis reaction are fed to a separation zone from which are discharged (i) from the top, methane and possibly hydrogen, (ii) a fraction consisting essentially of hydrocarbons with 2 and 3 carbon atoms per molecule, and (iii) from the bottom, a fraction consisting essentially of hydrocarbons with at least 4 carbon atoms per molecule, (c) only the fraction consisting essentially of hydrocarbons with 2 and 3 carbon atoms per molecule is fed to a steam-cracking zone, in the presence of steam, to transform at least a portion of the hydrocarbons with 2 and 3 carbon atoms per molecule to monoolefinic hydrocarbons; the fraction consisting essentially of hydrocarbons with at least 4 carbon atoms per molecule, obtained from the bottom of the separation zone, is supplied to a second hydrogenolysis zone where it is treated in the presence of a catalyst, the effluent from the second hydrogenolysis zone is supplied to a separation zone to discharge, on the one hand, hydrocarbons with at least 4 carbon atoms per molecule which are recycled at least partly to the second hydrogenolysis zone, and, on the other hand, a fraction consisting essentially of a mixture of hydrogen, methane and saturated hydrocarbons with 2 and 3 carbon atoms per molecule; a hydrogen stream and a methane stream are separated from the mixture and there is fed to the steam-cracking zone the hydrocarbons of the mixture with 2 and 3 carbon atoms, together with the fraction consisting essentially of hydrocarbons with 2 and 3 carbon atoms per molecule as recovered from the separation zone following the first hydrogenolysis zone. At the outlet of the steam-cracking zone are thus obtained, in addition to a stream of methane and hydrogen and a stream of paraffinic hydrocarbons with 2 and 3 carbon atoms per molecule, olefins with 2 and 3 carbon atoms per molecule and products with at least 4 carbon atoms per molecule. According to this U.S. Pat. No. 4,137,147 all C4+ compounds are further processed in the second hydrogenolysis zone.
WO2010/111199 relates to a process for producing olefins comprising the steps of: (a) feeding a stream comprising butane to a dehydrogenation unit for converting butane to butenes and butadiene to produce a dehydrogenation unit product stream; (b) feeding the dehydrogenation unit product stream to a butadiene extraction unit to produce a butadiene product stream and a raffinate stream comprising butenes and residual butadiene; (c) feeding the raffinate stream to a selective hydrogenation unit for converting the residual butadiene to butenes to produce a selective hydrogenation unit product stream; (d) feeding the selective hydrogenation unit product stream to a deisobutenizer for separating isobutane and isobutene from the hydrogenation unit product stream to produce an isobutane/isobutene stream and a deisobutenizer product stream; (e) feeding the deisobutenizer unit product stream and a feed stream comprising ethylene to an olefin conversion unit capable of reacting butenes with ethylene to form propylene to form an olefin conversion unit product stream; and (f) recovering propylene from the olefin conversion unit product stream.
WO2006/124175 relates to a process for conversion of a gas oil, vacuum gas oil and atmospheric residue to produce olefins, benzene, toluene and xylene and ultra low sulfur diesel which process comprises: (a) reacting the hydrocarbon feedstock in a fluid catalytic cracking zone to produce C4-C6 olefins and light cycle oil (LCO), (b) reacting the C4-C6 olefins in an olefin cracking unit to produce ethylene and propylene, (c) reacting the light cycle oil in a hydrocracking zone containing a hydrocracking catalyst to produce a hydrocracking zone effluent comprising aromatic compounds and ultra low sulfur diesel, and (d) recovering ethylene, propylene, aromatic compounds and ultra low sulfur diesel.
Conventionally, crude oil is processed, via distillation, into a number of cuts such as naphtha, gas oils and residua. Each of these cuts has a number of potential uses such as for producing transportation fuels such as gasoline, diesel and kerosene or as feeds to some petrochemicals and other processing units.
Light crude oil cuts such as naphtha and some gas oils can be used for producing light olefins and single ring aromatic compounds via processes such as ethane dehydrogenation in which the hydrocarbon feed stream is evaporated and diluted with steam and then exposed to a very high temperature (750° C. to 900° C.) in short residence time (<1 second) furnace (reactor) tubes. In such a process the hydrocarbon molecules in the feed are transformed into (on average) shorter molecules and molecules with lower hydrogen to carbon ratios (such as olefins) when compared to the feed molecules. This process also generates hydrogen as a useful by-product and significant quantities of lower value co-products such as methane and C9+ Aromatics and condensed aromatic species (containing two or more aromatic rings which share edges).
Typically, the heavier (or higher boiling point) aromatic species, such as residua are further processed in a crude oil refinery to maximize the yields of lighter (distillable) products from the crude oil. This processing can be carried out by processes such as hydro-cracking (whereby the hydro-cracker feed is exposed to a suitable catalyst under conditions which result in some fraction of the feed molecules being cracked into shorter hydrocarbon molecules with the simultaneous addition of hydrogen). Heavy refinery stream hydrocracking is typically carried out at high pressures and temperatures and thus has a high capital cost.
An aspect of such a combination of crude oil distillation and steam cracking of the lighter distillation cuts is the capital and other costs associated with the fractional distillation of crude oil. Heavier crude oil cuts (i.e. those boiling beyond ˜350° C.) are relatively rich in substituted aromatic species and especially substituted condensed aromatic species (containing two or more aromatic rings which share edges) and under steam cracking conditions these materials yield substantial quantities of heavy by-products such as C9+ aromatics and condensed aromatics. Hence, a consequence of the conventional combination of crude oil distillation and steam cracking is that a substantial fraction of the crude oil, for example 50% by weight, is not processed via the steam cracker as the cracking yield of valuable products from heavier cuts is not considered to be sufficiently high.
Another aspect of the technology discussed above is that even if only light crude oil cuts (such as naphtha) are processed via steam cracking a significant fraction of the feed stream is converted into low value heavy by-products such as C9+ aromatics and condensed aromatics. With typical naphthas and gas oils these heavy by-products might constitute 2 to 25% of the total product yield (Table VI, Page 295, Pyrolysis: Theory and Industrial Practice by Lyle F. Albright et al, Academic Press, 1983). Whilst this represents a significant financial downgrade of expensive naphtha and/or gas oil in lower value material on the scale of a conventional steam cracker the yield of these heavy by-products does not typically justify the capital investment required to up-grade these materials (e.g. by hydrocracking) into streams that might produce significant quantities of higher value chemicals. This is partly because hydrocracking plants have high capital costs and, as with most petrochemicals processes, the capital cost of these units typically scales with throughput raised to the power of 0.6 or 0.7. Consequently, the capital costs of a small scale hydro-cracking unit are normally considered to be too high to justify such an investment to process steam cracker heavy by-products.
Another aspect of the conventional hydrocracking of heavy refinery streams such as residua is that this is typically carried out under compromise conditions that are chosen to achieve the desired overall conversion. As the feed streams contain a mixture of species with a range of easiness of cracking this result in some fraction of the distillable products formed by hydrocracking of relatively easily hydrocracked species being further converted under the conditions necessary to hydrocrack species more difficult to hydrocrack. This increases the hydrogen consumption and heat management difficulties associated with the process. And also the yield of light molecules such as methane increases at the expense of more valuable species.
A result of such a combination of crude oil distillation and steam cracking of the lighter distillation cuts is that steam cracking furnace tubes are typically unsuitable for the processing of cuts which contain significant quantities of material with a boiling point greater than ˜350° C. as it is difficult to ensure complete evaporation of these cuts prior to exposing the mixed hydrocarbon and steam stream to the high temperatures required to promote thermal cracking. If droplets of liquid hydrocarbon are present in the hot sections of cracking tubes coke is rapidly deposited on the tube surface which reduces heat transfer and increases pressure drop and ultimately curtails the operation of the cracking tube necessitating a shut-down of the tube to allow for decoking. Due to this difficulty a significant proportion of the original crude oil cannot be processed into light olefins and aromatic species via a steam cracker.
US 2012/0125813, US 2012/0125812 and US 2012/0125811 relate to a process for cracking a heavy hydrocarbon feed comprising a vaporization step, a distillation step, a coking step, a hydroprocessing step, and a steam cracking step. For example, US 2012/0125813 relates to a process for steam cracking a heavy hydrocarbon feed to produce ethylene, propylene, C4 olefins, pyrolysis gasoline, and other products, wherein steam cracking of hydrocarbons, i.e. a mixture of a hydrocarbon feed such as ethane, propane, naphtha, gas oil, or other hydrocarbon fractions, is a non-catalytic petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
US 2009/0050523 relates to the formation of olefins by thermal cracking in a pyrolysis furnace of liquid whole crude oil and/or condensate derived from natural gas in a manner that is integrated with a hydrocracking operation.
US 2008/0093261 relates to the formation of olefins by hydrocarbon thermal cracking in a pyrolysis furnace of liquid whole crude oil and/or condensate derived from natural gas in a manner that is integrated with a crude oil refinery.
Steam cracking of naphtha results in a high yield of methane and a relatively low yield in propylene (propylene/ethylene ratio of about 0.5) as well as a relatively low yield of BTX, BTX is also accompanied by co-boilers of the valuable components benzene, toluene and xylenes which do not allow recovering those on-spec by simple distillation but by more elaborate separation techniques such as solvent extraction.
FCC technology applied to naphtha feed does result in a much higher relative propylene yield (propylene/ethylene ratio of 1-1.5) but still has relatively large losses to methane and cycle oils in addition to the desired aromatics (BTX).